With Node, you'd have to serialize data and pass it between child processes. And that really, really sucks. Haskell's parallelization story extends way past the "embarrassingly parallel" request handling.

A good data point based on a less contrived example is how handily Haskell web frameworks demolish Node.js at JSON serialization... and not much else (https://www.techempower.com/benchmarks/#section=data-r13&hw=...).

Finally, if you're looking to do something truly and extremely CPU-bound, I'd tell you to write it in a C derivative and bind it to Node or Haskell, regardless of what your favorite language is. Optimize for speed in some places, and programmer happiness in others. A one-size-fits-all approach isn't usually appropriate.